The present invention relates to a method and a system for remote detection of objects, each object being provided with a sensor comprising at least two magnetic elements arranged in a predetermined mutual relationship representing an identity of the sensor, wherein electromagnetic signals are generated for exciting the sensor elements to produce electromagnetic reply signals and wherein an amplitude of the electromagnetic reply signal from each sensor element is modulated by a magnetic field having a magnitude-variant and a magnitude-invariant component. The invention also relates to a sensor for remote detection of objects, comprising at least two magnetic sensor elements, which are arranged in a mutual relationship selected from a set of predetermined relationship and representing an identity of the sensor, or of an object to which the sensor is attached.
Many applications require a reliable and contactless detection of the presence, identity or position of objects within a detection zone. Common examples are for instance price labeling of commercial articles, identification of components in production lines, identification of material type at recycling plants or electronic article surveillance in e.g. shops.
For some applications it is sufficient to detect the presence of the object or article. One example is a simple electronic article surveillance system, which is arranged to provide an alarm signal, once a protected article is carried into a detection zone. Such a simple application uses one single sensor element in the form of a thin metal strip or wire with magnetic properties. The sensor element may be detected magnetically by means of arc-shaped magnetic generators/detectors, which expose the sensor element to a alternating magnetic field, which affects a physical property of the sensor element. Use is often made of the fact that the alternating magnetic field causes a periodical switch of the magnetic momentum of dipole of the sensor element, which is also known as Barkhausen jumps. Sensors of this kind are for instance disclosed in U.S. Pat. No. 5,496,611, EP-A-0 710 923 and EP-A-0 716 393.
A different single-element sensor technology is described in WO97/29463 and WO97/29464, wherein each sensor comprises a wire-shaped element of amorphous or nano-crystalline metal alloy. An important feature of the amorphous or nano-crystalline metal alloy is that the permeability thereof may be controlled by an alternating magnetic modulating field. Through a physical effect known as Giant Magnetoimpedance, the amplitude of an electromagnetic reply signal from the sensor is modulated by the magnetic modulating field, when the sensor is excited by an electromagnetic interrogation signal. The modulation in amplitude in the reply signal is detected and used for determining the presence of the sensor in the detection zone.
None of the electronic article surveillance applications described above provides a remotely detectable identity for each sensor. However, for advanced applications it is necessary to provide such identity information, representing e.g. an article number, serial number, material code etc for the respective object, to which each sensor is attached. Such sensors or markers are disclosed in WO88/01427, wherein each sensor or marker is provided with a number of magnetostrictive strips or ribbons made of an amorphous ferromagnetic material and arranged in predetermined angular relationships or at predetermined distances from each other. The identity of such a sensor is represented by the predetermined relationships as well as the respective type of individual sensor elements. The sensor elements are excitable to mechanical resonance by magnetic energy. The magnetic signals generated by the resonating sensor elements may be detected magnetically or inductively.
A similar system is described in WO93/14478, wherein the sensors or markers are provided with one or more than one electrical resonant circuits, each of which is inductively coupled to a respective magnetic sensor element. Each electrical resonant circuit is excited to oscillate electrically, and the resonant frequency thereof is controllable, through the permeability of the magnetic element, by an external magnetic field, wherein a simultaneous detection of several identical sensors is possible.
In summary, prior art sensors for remote detection of objects are either of a single-element type, allowing only the presence of each sensor to be detected, or of a multi-element type, allowing also an identity of each sensor to be detected. Single-element sensors are easier to design and produce and therefore have a lower unit cost. On the other hand, multi-element sensors require a supporting carrier (particularly for mechanically resonating sensor elements) and/or capacitive and inductive components (for the electric resonant circuit versions). Naturally, this implies a higher cost per unit. Additionally, since the multi-element sensors described above mainly operate by a magnetic or inductive link, the operating distance of the detection system is quire narrow.
An objective of the present invention is to provide a sensor for remote detection of objects, which is capable of representing an identity of the sensor, or of the object to which the sensor is attached, at a substantially lower cost than the prior art sensors. A subsidiary objective of the present invention is to provide a sensor, the operating distance of which is far better than that of the prior art multi-element sensors described above.
The objectives are achieved by a method for remote detection of objects, wherein each object is provided with a sensor comprising at least two magnetic elements arranged in a predetermined mutual relationship representing an identity of the sensor, wherein electromagnetic signals are generated for exciting the sensor elements to produce electromagnetic reply signals and wherein an amplitude of the electromagnetic reply signal from each sensor element is modulated by a first magnetic field having a magnitude-variant and a magnitude-invariant component.
The method further comprises the steps of generating a second magnetic field with rotating field vector; detecting a frequency shift in a component of said reply signal occurring when a magnitude-invariant component of said second magnetic field balances the magnitude-invariant component of said first magnetic field, wherein the respective sensor element is momentarily exposed to a resulting magnetic field with essentially no magnitude-invariant component; and determining an orientation of the respective sensor element from the orientation of the magnitude-invariant component of said second magnetic field, when said frequency shift occurs.
Furthermore, the objectives are achieved by a sensor for remote detection of objects, comprising at least two magnetic sensors elements, which are arranged in a mutual relationship selected from a set of predetermined relationships and representing an identity of the sensor, or of an object to which the sensor is attached, said sensor elements being electromagnetically detectable and comprising a magnetic material, the permeability of which is controllable by a magnetic field and the high-frequency impedance. of which depends on said permeability.
Other objectives, features and advantages of the present invention appear from the following detailed disclosure, from the drawings as well from the appended patent claims.